1. Field of the Invention
The present invention pertains to improvements in compositions and methods for zonal isolation for vertical, deviated, and horizontal oil and gas wells. The method involves the utilization of oil-base universal fluid for providing settable filter cake while drilling and the subsequent in-situ solidification of the drilling mud or both the drilling mud and universal fluid filter cake to compressive strengths well in excess of that required for casing support, zonal isolation, and borehole stability.
2. Description of Prior Art
The general procedure of drilling an oil or gas well includes drilling a borehole using a drilling fluid (also termed drilling mud). Subsequent to drilling the borehole, casing is run into the well and a cement slurry is then placed in the annulus between the outside of the casing and the borehole wall. In order to obtain a good cementing job, it is necessary for the cement slurry to displace substantially all of the drilling mud from the annulus. A reduced displacement efficiency arises from the geometry of the system and also from the fact that drilling fluids and cements are usually incompatible.
Nondisplaced mud (mud still left in the borehole after cementing) and mud filter cake are major causes of unsatisfactory cement performance. Since the nondisplaced mud and mud filter cake do not set or bond to the casing, the borehole wall or the set cement itself, the mud and filter cake do not support the casing properly and later can allow annular gas or fluid migration.
Wells frequently are drilled with oil-base muds which contain water as the internal emulsified phase. These oil-base muds are used, for example, to lower drilling torque and drag, reduce damage to productive formations, increase wellbore stability or reduce differential pressure pipe sticking, etc. The presence of oil mud in the wellbore can even further reduce the displacement efficiency while cementing with a conventional water-base cement slurry.
The drilling industry has sought to overcome the above problems by using a variety of techniques to displace the drilling fluid with cement, e.g., turbulent flow, casing movement (reciprocation/rotation), casing equipment (centralizers, flow diverters, mud cake scratchers), and special spacers and wash fluids, but these have had limited success. Even greater cementing difficulties are encountered when the boreholes deviate greatly from vertical. Major problems arise in connection with casing placement, drilled solids settling, mud displacement, casing centralization, fluid separation (free water), and mechanical friction. When a good cementing job is not obtained, it may be necessary to perforate the casing and squeeze cement under high pressure through the perforations into the annulus and try to fill the zones that were not properly cemented initially. Frequently, this squeeze cementing is not successful, and such failures may eventually lead to abandoning the hole.
One of the major objectives of a primary cementing is to obtain good zonal isolation in the annulus of the well. Effective zonal isolation is achieved by sealing the cement and borehole wall. The interface of the cement and borehole wall is usually an interface between the cement and drilling fluid filter cake which is the source of many cementing problems. Good zonal isolation can be achieved if the filter cake hardens, permanently bonds to the formation face and the cement, and provides hydraulic sealing.
Accordingly, the present invention is directed to overcoming the above-noted problems in the art and to providing a solution as more particularly described hereinafter.